feat: support Rolldown

[eltigerchino]

closes #13738

This PR:

• upgrades dev deps to usethe vite-plugin-svelte 6 pre-release
• adds vite 7 and vite-plugin-svelte 6 to Kit's peer dep ranges
• changes the enforced Vite config to support both https://github.com/vitejs/rolldown-vite and the current Vite release with Rollup.

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[changeset-bot]

🦋 Changeset detected

Latest commit: c3d2e95

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Minor

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[benmccann]

I filed an issue for a follow-up here: #13756

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-13747-svelte.vercel.app/

[sapphi-red]

preserveEntrySignatures is now supported with [email protected]. Let me know if there's any blockers on rolldown / rolldown-vite side.